Boreholes are drilled into the earth for many applications such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. In order to efficiently use expensive resources drilling the boreholes, it is important for analysts to acquire detailed information related to the geologic formations being drilled.
Various types of tools referred to as downhole tools may be conveyed through the boreholes to perform various types of measurements to provide the analysts with the needed information. One category of tools uses fiber-optic sensors with some of them containing Fiber Bragg gratings (FBGs) as the sensing element. In general, a series of gratings can be written into an optical fiber to produce a corresponding series of sensors. Typically, the FBGs are interrogated by illuminating the optical fiber with a tunable stable laser. Because the wavelength of light output by the laser is dependent on temperature, the wavelength of the laser light can fluctuate with temperature if the laser is disposed along with the FBGs in a borehole where temperatures can reach as much as 175° C. Such temperature changes can result in inaccurate or meaningless readings of the FBGs or in requiring a very complicated tunable laser system. Hence, it would be appreciated in the drilling and production industries if apparatus and associated method for interrogating FBGs were developed that could be deployed completely downhole and still accurately read the FBGs.